The present invention relates to two-stage radial compressors and to turbochargers that include such compressors, and more particularly relates to two-stage radial compressors wherein first-stage and second-stage impellers are arranged in a back-to-back fashion.
A two-stage radial compressor with back-to-back impellers is described in U.S. Pat. No. 6,062,028. In the embodiment shown in FIG. 2 of the ""028 patent, the pressurized air from the first-stage impeller passes through a diffuser into an interstage duct formed as a generally annular structure. From the interstage duct, the air passes through a row of deswirl vanes that reduce the tangential or swirl component of flow, and then enters the second-stage impeller. The pressurized air from the second-stage impeller goes into a second-stage volute that is concentrically arranged inside the interstage duct. To get the air out of the compressor housing assembly (which includes the interstage duct and second-stage volute), a discharge duct that leads from the second-stage volute out of the housing must penetrate through the interstage duct at one circumferential location of the duct. The presence of the discharge duct in the interstage duct causes the flow entering the deswirl vanes to be non-axisymmetric to a significant extent, because the fluid must flow around the outer surface of the discharge duct. Additionally, a significant amount of flow separation occurs as the flow comes off the discharge duct, and consequently substantial pressure losses are incurred.